Last weekend my TARC kids launched their rocket four times over the course of about 2 hours. The wind was moderate (5-8 mph) the whole time. The first two launches the rocket flew straight up. We were playing with the weight of the rocket to get the altitude. We were over the first flight, and under on the second. For the 3rd launch, we put in a weight that should have taken us between first two flights, but instead, the rocket veered hard as soon as it was off our (6 ft) rail, and flew well below our previous low flight. We made no change to the rocket other than adjusting the weight. We use BBs in a pouch positioned very close to the empty center of mass so that changing the weight shifts the CM by only tiny amounts. We chalked it up to a bad motor, and tried again with no changes. Pretty much the same thing happened, only slightly less severe. We flying Aerotech F42-8 motors on a 1 meter long rocket with a BT-80 body and four big fins. I'm not an experienced rocket person. Is this just the expected motor performance? Or should I look for a different problem?
Rocket flies straight and then won't
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How did the nozzles on the motors look? Nice and evenly eroded all the way around?
rcktnut
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Put the weight up higher. Tape to bottom of nose cone or put in tip of nose cone with clay. Can adjust weight in nose cone by removing/adding some after test. What is your current stability margin?
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It is also possible to have odd wind shears not far above ground level. We've seen our teams have similar issues. Watching the smoke trails shows distinct changes in wind speed and even direction. At finals last spring, there was a shear about 30-40' up. The rockets would hit that and turn hard left.
rharshberger
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Not yet ..but I would like to know more about this rocket beer..
SecondRow
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Not yet ..but I would like to know more about this rocket beer..
Me too. But they don’t allow them at our launches. 
You’re adding mass too close to the center of mass. That reduces the angular moment of inertia and allows less force to rotate the rocket, plus the slower speed off the rail makes your rocket easier to weather cock.
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Simply adding more mass cannot reduce the moment of inertia. If mass that was previously distributed away from the COM was brought closer to the COM, it would reduce it, but adding more mass at the COM won't reduce MOI. The relationship of MOI to mass will be different, but the MOI won't be reduced.
If it is weathercocking, having greater mass concentrated at the COM may still aggravate the tendency, though. It will take approximately the same amount of force to generate an angular acceleration, while the increased overall mass will have a greater tendency to resist translation. The rocket will be closer to being "pinned" in space at its COM, so when the lateral airspeed velocity pushes against the fins, it will tend more to rotate the rocket vs. just pushing the whole thing sideways. Combine that with a slower speed off the rail giving a more-lateral effective airspeed velocity, and it may indeed have a noticeable effect.
To your final point, they should also run their sim and examine the detailed output to see the instantaneous stability vs. altitude. It's possible it's not achieving good stability until a little way into free flight.
I interpreted Steve's comment to mean: adding mass close to the CM reduces the moment of inertia when compared to adding mass farther from the CM.
rharshberger
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We don't allow it during the launch....but in the evenings around the campfire....I have no idea what you may or may not be drinking....
That was exactly how I meant it but I sure phrased it wrong. Thanks for understanding. SolarYellow is right that it doesn’t reduce the angular moment of inertia.
I’m seeing similar issues with one of my TARC teams that is also using F42-8s. OpenRocket is showing around 58 ft/sec off the rail and a stability margin of 2.15. They will get 2 or 3 straight flights in a row and then one that will oscillate and/or arc over badly. Winds were relatively light between 3-7 mph. I was attributing it to wind gust/shear, but I’m starting to wonder if the fast burning blue thunder propellant is more prone to thrust asymmetries than other motors.
So when you say "but instead, the rocket veered hard as soon as it was off our (6 ft) rail, and flew well below our previous low flight." I'm thinking that maybe the lower rail button may have hung up.. Maybe some soot from the previous flights.... ? I don't know for sure, but I think that I would've put the model back to the same specs as the first successful flight and tried again. Especially since you say that the next flight basically did the same thing.
Just my thoughts.. I may be totally off the mark, but I tend to look at the simple "solution" first.
Just my thoughts.. I may be totally off the mark, but I tend to look at the simple "solution" first.
What is your thrust-to-weight ratio? Is it at least 3:1, preferably 5:1 or more? Stability margin at least 1.5-2 cal?
What is the velocity of the rod/rail? You can determine this with a simulation.
What is the velocity of the rod/rail? You can determine this with a simulation.
THREE questions . . .
(1) What was your total liftoff mass on the first flight ?
(2) What was the maximum amount you increased it by ?
(3) What is your Margin of Stability, in Calibers ?
The PDF files below may be helpful . . .
What he said. I wouldn't bet on this as the problem, but it's really easy check and eliminate.
Another thing that seems unlikely in my mind, but is easy to check. Check the rail buttons while you're at it.
Totally on the mark. If A succeeds and B fails, and you think you've changed only one thing, go back to A just to make sure there's nothing else that changed that you didn't know.
Maybe not what he said. If weather cocking is the problem (and that's still not known) then adding nose weight can make it worse, not better. If the problem is insufficient static margin, the yes, nose weight is a good answer. You have to know what you're dealing with before you can take action to fix it. Sometimes "try it and see" is a way to find out, but it's usually not a good way.
What does the rocket weigh? (Someone asked that already.) I just looked, and I see that the F42 is a very punchy motor, so unless you're really heavy I doubt that rail exit speed is the problem. And since this happened with the middle weight configuration, not with the even heavier one, I'd bet good money that that's not the problem. But then, if you've got a lot of rail exit speed then weathercocking is less likely, even though that's what this sounds like otherwise.
Rocket Science is hard. Drinking rocket beers around the campfire is easy!
The CCP has experienced "unfortunate gusts of wind" that have officially caused thier rockets to take out entire towns. So I would just stick to the PARTY 'S line. You will not find the party. The PARTY will find you! Oh to be back in high school again. The early 80's rocked!
The CCP has experienced "unfortunate gusts of wind" that have officially caused thier rockets to take out entire towns. So I would just stick to the PARTY 'S line. You will not find the party. The PARTY will find you! Oh to be back in high school again. The early 80's rocked!
I'm betting on a model with a large margin of stability ( over-stable ), combined with large fins.
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I have witnessed similar launch events where an earlier flight flew perfect. The second flight of the day went unstable and crashed. Same rocket, same type motor, the only difference was the 1010 rail. My opinion on the cause was excessive rail drag. An issue that is starting to gain interest for myself.
Whether it is an anomalous motor performance, low off the rail velocity from rail or rod drag, or low ground wind shear all of these I have experienced and recorded on a high speed flight computer. Since this was for a TARC competition, did you have avionics on these anomalous flights? If you had avionics did any anomalous data points appear in the data?
T/W ratios for launch rod and rail velocities are a quick rule when things go well. I have noticed on my flights that the on-board flight computer recorded lower off the rod launch velocities than what Open Rocket and RockSim predict. I can only assume that the simulation software available does not account for launch rod or rail drag. An issue that is not well understood at this time and not predictable.
Off the rail pitch and yaw issues are best studied with 3-axis gyro data. The motor anomalies I’ve recorded show up in the accelerometer data. I’ve included a few examples.
Whether it is an anomalous motor performance, low off the rail velocity from rail or rod drag, or low ground wind shear all of these I have experienced and recorded on a high speed flight computer. Since this was for a TARC competition, did you have avionics on these anomalous flights? If you had avionics did any anomalous data points appear in the data?
T/W ratios for launch rod and rail velocities are a quick rule when things go well. I have noticed on my flights that the on-board flight computer recorded lower off the rod launch velocities than what Open Rocket and RockSim predict. I can only assume that the simulation software available does not account for launch rod or rail drag. An issue that is not well understood at this time and not predictable.
Off the rail pitch and yaw issues are best studied with 3-axis gyro data. The motor anomalies I’ve recorded show up in the accelerometer data. I’ve included a few examples.
John Taylor
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If the CG is too far forward of the CP
(OVERSTABLE) the rocket is MUCH more prone to turning into the wind
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Come on, it’s just like military flightline rules.
1. No smoking within 24 hours of the flight.
2. No drinking within 50 feet of the vehicle.
Unless, like… you ARE the party. Then… uhh… the party… is always… dude, we need some pizza.
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